Editing Exosphere (section)

===Lower boundary===
{{Main|Thermopause}}
The lower boundary of the exosphere is called the ''thermopause'' or ''exobase''. It is also called the ''critical altitude'', as this is the altitude where [[Barometric formula|barometric conditions]] no longer apply. Atmospheric temperature becomes nearly a constant above this altitude.<ref>Bauer, Siegfried; Lammer,  Helmut. ''Planetary Aeronomy: Atmosphere Environments in Planetary Systems'', [[Springer Publishing]], 2004.</ref> On Earth, the altitude of the exobase ranges from about {{convert|500|to|1000|km|lk=on}} depending on solar activity.<ref>{{cite web |title=Exosphere - overview |url=http://scied.ucar.edu/shortcontent/exosphere-overview |date=2011 |publisher=UCAR |access-date=April 19, 2015 |archive-url=https://web.archive.org/web/20170517071138/https://scied.ucar.edu/shortcontent/exosphere-overview |archive-date=17 May 2017 |url-status=dead }}</ref>

The exobase can be defined in one of two ways:

If we define the exobase as the height at which upward-traveling molecules experience one collision on average, then at this position the [[mean free path]] of a molecule is equal to one pressure [[scale height]]. This is shown in the following. Consider a volume of air, with horizontal area <math>A</math> and height equal to the mean free path <math>l</math>, at pressure <math>p</math> and temperature <math>T</math>. For an [[ideal gas]], the number of molecules contained in it is:
: <math> N = \frac{pAl} {k_{B}T} </math>

where <math>k_B</math> is the [[Boltzmann constant]]. From the requirement that each molecule traveling upward undergoes on average one collision, the pressure is:

: <math> p = \frac{m_{A}Ng} {A} </math>

where <math>m_{A}</math> is the mean molecular mass of the gas. Solving these two equations gives:

: <math> l = \frac{k_{B} T} {m_{A}g} </math>

which is the equation for the pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and because the [[Knudsen number]] is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where <math>\mathrm{Kn}(h_{EB}) \simeq 1</math>.

The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from [[orbit]] if no action is taken to maintain the orbit.